Some browsers don't support TrackElement

dart/tests/html/track_element_constructor_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // A regression test for dart2js generating illegal JavaScript code
 // dynamically in non-csp mode.  The name of the field "defaultValue"
 // in JavaScript is "default".  This meant that dart2js would create a
 // constructor function that looked like this:
 //
 // function TrackElement(default) { this.default = default; }
 
 import 'dart:html';
 
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 
 void main() {
   useHtmlConfiguration();
   test('', () {
+    if (!TrackElement.supported) return;
     document.body.append(new TrackElement()..defaultValue = true);
     if (!document.query('track').defaultValue) {
       throw 'Expected default value to be true';
     }
     document.query('track').defaultValue = false;
     if (document.query('track').defaultValue) {
       throw 'Expected default value to be false';
     }
   });
 }